Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface showing a first embodiment of our new design; and,

FIG. 2 is a front view of an alternate embodiment thereof.

The dash-dot broken lines in the drawings illustrate the display screen or a portion thereof while the even broken lines illustrate portions of the graphical user interface. No subject matter in either broken lines form part of the claimed design. 

CLAIM We claim the ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 